Electrolytic recording medium

ABSTRACT

An improved electrolytic recording medium for use with dissolving electrode type of facsimile recording apparatus is disclosed comprising a porous sheet impregnated with an aqueous medium including a marking compound, an electrolyte and an effective amount of a compatible, fluorescent brightening agent.

United States Patent Diamond et al.

[15 j 3,668,079 [451 June 6,1972

[54] ELECTROLYTIC RECORDING MEDIUM [72] Inventors: Arthur S. Diamond, Palos Verdes Peninsula; David E. Carr, Playa Del Rey, both of Calif. [73] Assignee: Telautograph Corporation, Los Angeles,

Calif. [22] Filed: May 13, 1971 211 App]. No.: 142,902

[52] U.S. Cl ..204/2, 1 l7/20l [5 l] Int. Cl ..B21h 1/20 [58] Field ofSearch ..204/2; ll7/20l,2l6

[56] References Cited UNITED STATES PATENTS 3,216,855 1 1/1965 Ellison ..204/2 Gradsten et al ..204/2 3,342,705 9/1967 Lieblich et a] 204/2 3,344,043 9/1967 Lieblich et al. .....204/2 3,344,044 9/ l 967 Gradsten .204/2 3,5 l5,648 6/1970 Chiu ..204/2 Primary Examiner.lolin H. Mack Assistant Examiner-T. Tufariello Attorney-Lindenberg, Freilich & Wasserman [57] ABSTRACT An improved electrolytic recording medium for use with dissolving electrode type of facsimile recording apparatus is disclosed comprising a porous sheet impregnated with an aqueous medium including a marking compound, an electrolyte and an effective amount of a compatible, fluorescent brightening agent. I

12 Claims, 3 Drawing Figures ELECTROLYTIC RECORDING MEDIUM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electrolytic recording media, and, more particularly, to an improved, wet, electrolytic recording paper for use with dissolving-electrode type, facsimile recording apparatus.

2. Description of the Prior Art Recording paper of the wet electrolytic type is generally marked by passing the paper between a positive eroding metal anode electrode and a negative non-eroding cathode electrode. The paper is impregnated with an electrolytically conductive solution containing a marking ingredient, one or more electrolytes and various other stabilizing and image enhancing additives. When a voltage is applied between the electrodes causing current to flow through the recording paper, metal ions dissolve at the anode, enter the wet paper and react with the marking ingredients of the impregnant to form a colored mark on the side of the paper in contact with the anode.

For facsimile recording purposes, a white paper is preferred; it should have the ability to produce marks having an optical density which bears a close linear relationship to the amount of current passed through the paper. The density of the mark area should be variable uniformly in a range from white for zero current flow through greys for current of intermediate magnitude to black for currents of maximum flow. A black mark is preferred to provide a good contrast with the white background. 1

There should also be a minimum of bleeding or fringing of the marks so as to provide good resolution. The unmarked paper while stored in a moist condition in sealed containers for extended periods of time prior to use, should remain stable without chemical decomposition of its ingredients which might cause discoloration of the paper or affect the reproducibility of results without recorder adjustment. Further, the marked paper should not become discolored, give off unpleasant odors, release any substance to the environment, or transfer its mark to adjacent materials. The marked recording paper should be relatively insensitive to light even after repeated runs through any of the conventional copying machines involving exposure to ultraviolet or other strong light sources. I

Electrolytic recording papers are currently manufactured and sold for use with iron, silver, or other metallic marking electrodes. These devices are most commonly of the helixand-bar, single-stylus, or multiple stylus type. All produce a visible image when electrical current flows trough the recording medium positioned between a printer blade or stylus anode and a noneroding cathode backing member.

Electrical impulses cause the anode metal to dissolve in the wet, impregnated paper. A color-forming reaction then occurs between the dissolved metal ions and the marking compound incorporated in the wet, conductive paper.

Marking compounds in common use include catechol (pyrocatechol) which forms an intense black mark with ferrous ions and methylene disalicyclic acid which produces a sepia-colored species upon reaction with ferrous ions. Other compounds used in electrolytic recording papers include reducing agents such as ascorbic acid and sodium formaldehyde sulfoxylate which produce a black mark in the presence of silver ions.

.The color-forming tendencies of these marking compounds particularly in the presence of metal ions, place an unusually high demand on the purity of papers intended for impregnation and especially in the case of catechol based electrolytic marking papers. Iron, copper, titanium, and formaldehyde are four main contaminants to be avoided in papers intended for electrolytic recording. Yet, these substances are found in almost all papers in greater or lesser amounts.

For acceptable shelf life in catechol papers, iron and copper content must be kept below about 20 parts per million each, or a total contamination less than about 35 parts per million.

This requirement rules out the vast majority of paper mills in the United States owing to the high mineral content of most process water streams and the presence of copper bearing alloys in existing pipes, valves, and fittings.

Titanium contamination becomes a problem usually when titanium dioxide filled papers are runon the same machine as the electrolytic base paper. Unless the paper machine is cleaned up thoroughly, titanium might be transferred to the electrolytic grade. This metal causes a reddish background color to develop in catechol papers under natural or accelerated storage conditions. v

Although formaldehyde is present in the high wet strength papers used for iron stylus applications, it is combined with urea or melamine as a wet-strength resin. Only free formaldehyde appears to affect shelf life; bound formaldehyde in melamine and urea formaldehyde resins seems to be compatible with the polyhydroxy phenolic marking compounds.

The foregoing requirements for a high-purity base paper severely limit the number of acceptable sources in this country. They also cause a substantial price increase in this paper over comparable grades, as a high percentage of paper manufactured specifically for electrolytic recording is rejected by quality control test procedures. I

SUMMARY or THE lNVEN'llON In accordance with the invention, an electrolytic recording medium intended for electrolytic facsimile recording is provided that is whiter and brighter than previous electrolytic recording media of the same type. The useful shelf life of the impregnated recording medium of this invention is considerably'extended by substantially reducing its tendency to discolor with age. The electrolytic recording impregnating solution of this invention enables lower purity base papers to be used in the manufacture of electrolytic recording papers by lowering the tolerances on iron and copper contamination levels. Substantial cost savings are effected by permitting the use of lower purity base papers and by extending the storage life'of these products. Another advantage of the medium of the invention is that the eveness of impregnation can be readily tested and observed under ultraviolet'(b lack light") illumination.

These and many other attendant advantages of the invention will become apparent as the description proceeds.

An improved electrolytic medium in accordance with the invention is provided by impregnating a porous base sheet with an aqueous electrolytic recording solution containing a marking compound, an electrolyte and an effective amount of a stable and compatible, fluorescent-brightening agent. The medium is found to exhibit substantially increased brightness and whiteness and to inhibit the tendency to color on aging. In addition to its optical. whitening effect, the preferred brighteners of invention are capable of inhibiting discoloration in storage due to metal contaminants in the base paper.

A recording paper on aging will typically develop a yellow or brown background color. The spectral reflectance of the aged paper is defective in the blue region. The human eye is only sensitive to a narrow band of radiant energy from 4,000 to 8,000A. It is not sensitive to ultraviolet or infrared wavelengths. However, sunlight and electrical illumination sources contain a significant amount of ultraviolet radiation. The presence of fluorescent brightening agents within the impregnant absorb invisible radiation at a wavelength below 4,000A and emit visible radiation preferably having a maximum peaking within the blue region of the visible spectrum. The presence of the agent does not significantly affect the total radiation reflected by the fabric but by converting invisi ble U.V. radiation into visible light, the agent makes the recording paper appear both brighter and whiter.

The invention will now become understood by reference to the following detail description when considered in conjunction with the accompanying drawings. 1

BRIEFTDIESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a process for preparing an improved electrolytic recording medium in accordance with the DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, a sheet of high wet strength, white paper or other porous base is fed from roll 12 and passes over turning roll 14 into the liquid electrolytic recording solution l6 contained within an impregnating tank 18. The web travels past submerged rolls and 22 and absorbs the impregnating solution which distributes itself evenly throughout the sheet 10. The impregnated sheet 10 is turned by roll 24 and passesbetween pressure rollers 26 and 28 which adjust the water content in the sheet 10 to about to 50 percent by weight on a wetbasis. The evenessof distribution of the impregnant can be tested by projecting a beam of UV. light on thesheet and observing the fluorescent pattern. The impregnated sheet 10 is rewound on takeup roller 30. The sheet is then cut into smaller rolls 32 and is packaged and stored in a suitable, sealed container such as the heat sealed polyethylene bag 33 depicted in FIG. 2.

Referring 'now to FIG. 3, the impregnated sheet 10 is recorded by passing it between a stationary, noneroding anode 34 and a rotatable, noneroding cylindrical cathode 36 which has a prismatic helical electrode 28'wound around the projecting above its exterior surface. The electrode 38 sweeps a spot longitudinally across a knife-edge 40 of the eroding anode 34. Current passing between the anode 34 and the cathode 36 causes the anode to erode electrolytically and causes metal ions to enter the electrolytic solution with which sheet 10 has been impregnated'The ions are either reduced to free metal by the marking ingredients, or enter into a color-forming reaction with chemicals in the paper and leave a dense black mark where the paper is contacted simultaneously on opposite side by the anode and the cathode electrodes.

l The'eroding anode electrode may be made of any suitable material such as iron, stainless steel, silver or copper together with alloys of various metals as is well known in the art. For recording with a polyhydroxy phenolic marking compound such as catechol, the anode is preferably made of an alloy of iron,'such as stainless steel. The cathode electrode may be made of any suitable,-conductive metal capable of withstanding the abrasive effects of the moving recording paper. Metals found acceptable are platinum, platinum alloys, stainless steel and othermaterials well known in the art. The cathode is suitably formed of a platinum-iridium alloy.

Polyhydroxy phenolic marking ingredients are suitably catechol, methylene disalicylic acid (MDA) protocat'echuic acid (PCA) dihydroxyphenylacetic acid (DHPAA), gallie acid, dihydroxy-phenyl-propionic acid (DHPPA) or any of the polyhydroxy phenolic marking ingredients disclosed in U.S. Pat. Nos. 3,344,043, 3,342,704 and 3,342,705, the disclosure of which is incorporated herein by reference. The marking ingredient is generally maintained in an amount between 20 to 100 grams and preferably between 30 and grams per liter of solution.

The electrolytic salt is usually a nitrate, chloride or sulfate Of an alkali metal or of ammonium. The salt is generally present in an amount between about 50 to 150 grams per liter of a solution.

a The inclusion of an oxidizing agent such as taught in U.S.

Pat. No. 2,953,505 to Mones speeds up the color-fomiing processes in the absence of heat, increases the sensitivity of the medium and provides greater marking density. Suitable oxidants are alkali metal chlorates, bromates, perchlorates, iodat'es and periodates. Suitably, sodium chlorate is present in the medium. in an amount from 20 to 150 grams per liter'of solution.

Other additives such as organic or inorganic acids may be provided to maintain a low pH between about 2.0 and 3.0, thereby'preventing premature color formation. Suitable acids are at least one of phosphoric acid, oxalic acid, tannic acid and formic acid. The acids are suitably present in the medium in an amount below 10.0 grams per-liter of solution and at a level necessary to achieve the desired pH.

Other ingredients such as thiourea, alkyl derivatives of thiourea or dithiobiuret may be present in small amounts, suitably below 20 grams per liter of solution to stabilize the recorded mark. Metal sequestering agents such as organic polyarnines, suitably EDTA and an image tone modifier such as an alkali metal phosphate may be added to the impregnating solution. The phosphates are typically present in an amount from 1.0 to 10.0 grams and preferably 2.0 to 4.0 grams per liter of solution.

For iron stylus recording papers, the paper base is preferably a light weight, low pH, high. wet-strength base paper. These papers may be prepared by treatment with a wet strength resin such as an amine-formaldehyde resin. Typically, a melamine-formaldehyde wet strength resin is added to the head box of the paper machine. The paper should have an extracted pH below 7," preferably about 5.5. These papers are available from commercial sources, a suitable example being Paterson Hi-Wet 600.

For silver stylus recording papers, the paper base is preferably a light weight, neutral or alkaline, high wet-strength base paper. These papers may be prepared by treatment with special agents such as cationic, polyamide-epichlorohydrin resin, suitably DuPonts Quilon, or a Werner-type chromium complex in isopropanol such as Hercules Kymene," which is approximately a 30 percent solution of Kymene stearata chromium chloride or other suitable wet strength agent which does not cause a highly acid condition in paper. The silver-marking papers should have an extracted pH above 7.0, preferably between 7.0 and 9.0. Base papers are available from commercial sources, a suitable example being Kimberly- Clarks alkaline facsimile base paper. i

The fluorescent brightening agent is preferably a water soluble, acid stable nitrogen containingorganic material that is compatible with theingredients of the medium and with the operation of electrolytic facsimile recording machines. For example, many recorders contain a heating section for more rapid drying of the recorded paper. The agent must be capable of withstanding temperatures of up to about 200..F, without decomposing or discoloring the background of the marked areas of the sheet. The agent should be stable to light and not cause any discoloration of the paper on extended storage. The

agent should exhibit substantial absorption at wavelengths below 4,000A and besubstantially free of groups absorbing at wavelengths above 4,000A.

A controlled amount of the fluorescent agent can be added to the pulp slurry in the beater prior to the formation of the paper sheet at the paper mill or the agent can be added to the impregnating solution. Generally, l to 10 grams of the agent is added to a liter of solution of the impregnant. Preferably, between 2.0 and 5.0 grams per liter are used. High concentrations appear to result in a decrease in brightness possibly due to brightener precipitation, loss of the fluorescent advantage due to a'buildup of excessive visible color, or, in the case of iron-marking papers premature coloration due to an increase in pH caused by the alkaline brightener material. The agents are compatible with electrolytic salts and in fact the salt may participate in the transfer of the agent to the cellulose fibers of the paper.

. Nitrogen atoms present in the fluorescent brighteners of this invention are capable of fonning ligand structures with iron,

copper, and other metals which might be present as contamimetal contaminants present in the base paper in the range of 0 to 25 parts per million, thereby forming coordination com- SOJH H033 where R,, R R and R are selected from the group consist- 5 ing of lower alkyl of one to eight carbonatoms such as methyl or ethyl, lower alkoxy of 1-8 carbon atoms such as ethoxy or propyloxy, aryl such as phenyl, aralkyl such as benzyl, amino, substituted amino such as N-alkylamino, N-phenyl amino, N- ethoxy-amino and sulfonated derivatives thereof.

Exemplary compounds of the formula include 4:4-bis(4 ":6' '-di-B-hydroxyethoxy-l ":3":5-tria.zin-2"-ylarnino) stilbene-2:2'-disulfonic acid; 4:4'-bis(4":6"-di-B-hydroxypropoxy-l ":3":5"-triazin-2 '-ylamino) stilbene-2:2'-disulfonic acid; and 4"4-bis(4":6"-diamine l:3":5"-triazin-2- ylamino) stilbene-2"2-disulfonic acid.

The following examples are presented solely for the purposes of specially illustrating the invention and are not intended to in any way limit the scope of the invention.

The following masterbatch impregnating solutions were formulated as follows:

The above ingredients, when mixed together, give a clear solution having a pH equal to 2.3.

Sodium Nitrate 46.0 gm Sodium Chlorate 94.0 gm Formic Acid 2.0 ml vCatechol 50.0 gm Deionized water to make 1000.0 ml

The above ingredients, when mixed together, gave a clear solution having a pH equal to 2.2.

EXAMPLE 4 0.5 ml of the P,P-diamino, 0,0'-disulfonic acid (Uvitex PBSA produced by ln'tracolor Corp.) were added to two 100 ml portions of the masterbatches of Examples 1-3 to prepare 0.5 percent solution by volume with respect to-brightening agent indicated as Examples l-4, 2-4 and 3-4. These solutions and two control solutions for each masterbatch absent the agent, were impregnated onto paper and the moisture content was adjusted to 40 percent. The papers were heat aged in an oven at 65 C for 42 hours. The reflectance as measured with a Photovolt reflection densitometer was recorded before aging (R and after aging (R The results are recorded in the following table. It is to be noted that in this heat aging test, an R value of 70.0 is the minimum acceptable; it represents a shelf life of 6 months.

The brightening agent in each case raised the brightness value of the impregnated paper as compared to the controls. The papers glowed brightly in the dark under U.V. radiation showing eveness of impregnation. Dark streaks were indicated in drier parts of the paper. Heat aging at 65 C did not destroy the effect of the brightening agent. Y

EXAMPLE 5 A 10 percent solution of the fluorescent agent in water was prepared and 0.05 ml (B) 0.l ml (C), 0.5 ml (D) and 1.0 ml (E) portions were added to 100 m1 of the masterbatch of Example 3 while one 100 ml portion (A) served as a control. The portions were impreganted onto a paper base and heat aged as previously described. The brightness is recorded in the follow- EXAMPLE 2 mg table where all values are well above the 70.0 minimum.

TABLE 11 Component Amount Thiourea 2.4 gm Example R, R4, Oxalic Acid 1.0 gm Sodium Phosphate l.0 gm Sodium Nitrate 46.0 gm 5A 78 76 Sodium Chlorate 99.0 gm 58 BI 77 Formic Acid 2.0 ml 5c 81 71.5 Catechol 48.0 gm 5D 81 79 Deionized Water to make 1000.0 ml 5E 82 79 The above ingredients, when mixed together, gave a clear Paper brightness in the wet, fresh state and in the heat aged solution having a pH equal to 2.3.

EXAMPLE 3 state was considerably increased by the presence of the agent.

EXAMPLE 6 Three ml portions of the masterbatch of Example 3 were prepared and three impregnating solutions were prepared containing by volume, 0 percent brightening agent, 0.1 percent brightening agent, and 0.25 percent brightening agent. These solutions were impregnated onto abase paper containing 15 ppm copper and onto papers containing increasing amounts of copper. Reflectance values after heat aging are reported in Table III below.

- it is observed that increasing amounts of brightener increase the whiteness or brightness of the paper both fresh (R and alter 42 hours of heat aging (R Also,increasing amounts of brightener counteract the discoloration brought on by higher levels of copperpresent as a contaminant in the paper. The brightener appears ineffective above about 25 parts per million of copper. .At concentrations above about 0.50 percent, by volume, of brightener, the solution became turbid and some precipitate was formed in the bottom of the mixing chamber. i I

I EXAMPLE7 Component Amount Oxalie Acid 1.6 gm' Thiourea 3.5 gm Sodium Phosphate 1.0 gm Sodium Chlorate 40.0 gm Formic Acid 2.0 ml Sodium Nitrate 60.0 gm Ammonium Thiocyanate 60.0 gm Gallic Acid 40.0 gm Ethyl Alcohol 200.0 ml Propylene Glycol 27.0 ml

Deionized water to make 1000.0 ml

, The above ingredients were mixed together to produce a clear solution having a pH of 2.2. This solution was used to impregnate samples of Paterson Hi-Wet 600 and Crane Facsimile Base Paper, both high wet-strength papers. Good recordings were obtained on both United Press International s Unifax" and Associated Presss Photofax photofacsimile receivers using a steel printer blade.

' EXAMPLE 8 Same as Example 7, except2.$ ml of Calcofluor H, (America Cyanamid) a triazinyl stilbene type, anionic, fluorescent brightener were added to the formulation. The resultant prints were much whiter, brighter, and more contrasty than obtained in Example 7. Also, for some reason the image appeared to be smoother, more uniform and less grainy than prints 7 reproduced by wire photo transmission in Example 7. This improvement in image tone and uniformity is not wholly understood, however it could be the result of some mordant effect the brightening agent has whereby it causes the marking compound to be deposited preferentially on the surface of the facsimile base paper rather than being distributed uniformly throughout the body ofthe sheet.

Y EXAMPLE 9 i Same as Example 7, except 2.5 ml of Uvitex PBSA (Intracolor Corporation) were added to the formulation. Results 1 were the same-as those obtained in Example 8.

EXAMPLE 10 Component Amount Potmium Nitrate 80.0 gm Potassium Carbonate 4.2 gm Sodium Formaldehyde Sulfoxylate (NOPCO Hydrosulfite AWC) 53.3 gm Sodium Stannate 1.7 gm Triton X-lOO (10% solution) 13.5 ml Deionized water to make 1000.0 ml

The above ingredients, when mixed together, gave a pH equal to 10.9. A sheet of (kane 33-91" alkaline facsimile base paper measuring 8 l X 30 inches was impregnated with 50 ml of this solution and then used for facsimile recording on a Telautograph Quikfax Model 900 receiver having a silver blade. The actual blade composition was 75 percent silver, 22 percent copper, and 3 percent zinc. Excellent recordings were obtained.

, EXAMPLE l1 To ml of the solution prepared under Example 9 above was added 0.25 ml of Calcofluor H (America Cyanamid) fluorescent brightener. This solution containing optical brightener was then used'to impregnate a second sample of Crane 33-91 paper in the same manner asExample 9. The

recording obtained was equal to clarity and density but had a brighter and whiter background appearance. EXAMPLE 12 Same as Example ll except using Tinopal 2BA (Geigy) optical bleach. Similar results were obtained. 7

It is to be understood that only preferred embodiments of the invention have been described and that numerous substitutions, modifications and alterations are all permissible without departing from the spirit and scope of the invention as defined in the following claims.

What is claimed is:

1. An electrolytic recording medium for marking a dissolving metal anode blade comprising a porous sheet impregnated with an aqueous electrolytically conducting marking solution containing:

an electroresponsive marking compound;

an ioni zable electrolytic salt; and w an effective amount of an organicfluorescent brightening agent compatiblewith said medium.

2. A recording medium according to claim 1 in which the marking compound is a polyhydroxy phenolicmarking compound present in said solution in an amount between 20 to 100 grams per liter of solution. u i v 3. A recording medium according to claim 2 in which the porous sheet isa light weight, high wet strength paper having an extracted pH below about 7 and the marking compound is selected from the group consisting of catechol, methylene disalicylic acid, protocatechuic acid, dihyrdoiryphenylacetic acid, gallic acid and dihydroxyphenylpropionic acid and is present in an amount between 2.0 to 5.0 grams per liter of solution. v

4. A recording medium according to claim 1 in which the electrolytic salt is selected from nitrates, chlorides and sulfates of a member selected from the group consisting of ammonium and alkali metals. I

5. A recording medium according to claim 1 further including 20 to grams per liter of solution of an oxidin'ng agent.

6. A recording medium according to claim 5 in which the oxidizing agent is an alkali metal salt selected from chlorates, bromates, perchlorates, iodates and periodates.

7. A recording medium according to claim 1 in which the sheet contains from 25 to 50 percent water.

8. A recording medium according to claim 1 in which the fluorescent brightening agent is a water soluble, acid stable,

fluorescent brightening agent is selected from compounds of the formula:

3 3E H038 trolytic salt and an organic, fluorescent brightening agent;

1 N N contacting a first surface of the sheet with a dissolving metal N NH-CH=CH-- H \I anode and the obverse surface with a nondissolving =K cathode; and

passing current through said electrodes to dissolve and ionize said anode and react the ions with said compound where R R and R R are selected from the group consistto form a mark. mg of lower alkyl of one to eight carbon atoms, lower al- V koxy of 1-8 carbon atoms, aryl, aralkyl and amino. 12. A method according to claim 11 in which the anode 11. A method of forming a mark on an electrolytic recordcomprises iron, the compound is a polyhydroxy phenolic coming medium comprising the steps of: pound and the agent is a P,P'-diamino stilbene 0,0'-disulfonic impregnating an absorbent sheet with an aqueous sheet cona taining an electroresponsive marking compound, an elecw a it e i 

2. A recording medium according to claim 1 in which the marking compound is a polyhydroxy phenolic marking compound present in said solution in an amount between 20 to 100 grams per liter of solution.
 3. A recording medium according to claim 2 in which the porous sheet is a light weight, high wet strength paper having an extracted pH below about 7 and the marking compound is selected from the group consisting of catechol, methylene disalicylic acid, protocatechuic acid, dihyrdoxyphenylacetic acid, gallic acid and dihydroxyphenylpropionic acid and is present in an amount between 2.0 to 5.0 grams per liter of solution.
 4. A recording medium according to claim 1 in which the electrolytic salt is selected from nitrates, chlorides and sulfates of a member selected from the group consisting of ammonium and alkali metals.
 5. A recording medium according to claim 1 further including 20 to 150 grams per liter of solution of an oxidizing agent.
 6. A recording medium according to claim 5 in which the oxidizing agent is an alkali metal salt selected from chlorates, bromates, perchlorates, iodates and periodates.
 7. A recording medium according to claim 1 in which the sheet contains from 25 to 50 percent water.
 8. A recording medium according to claim 1 in which the fluorescent brightening agent is a water soluble, acid stable, nitrogen containing organic compound present in said medium in an amount between 1 to 10 grams per liter of solution.
 9. A recording medium according to claim 8 in which the fluorescent brightening agent is present in the medium in an amount between 2.0 to 5.0 grams per liter of solution and is a P, P''-diamino stilbene 0,0''-disulfonic acid.
 10. A recording medium according to claim 9 in which the fluorescent brightening agent is selected from compounds of the formula: where R1, R2 and R3, R4 are selected from the group consisting of lower alkyl of one to eight carbon atoms, lower alkoxy of 1-8 carbon atoms, aryl, aralkyl and amino.
 11. A method of forming a mark on an electrolytic recording medium comprising the steps of: impregnating an absorbent sheet with an aqueous sheet containing an electroresponsive marking compound, an electrolytic salt and an organic, fluorescent brightening agent; contacting a first surface of the sheet with a dissolving metal anode and the obverse surface with a nondissolving cathode; and passing current through said electrodes to dissolve and ionize said anode and react the ions with said compound to form a mark.
 12. A method according to claim 11 in wHich the anode comprises iron, the compound is a polyhydroxy phenolic compound and the agent is a P,P''-diamino stilbene O,O''-disulfonic acid. 